As a particulate matter detection sensor (hereafter, also referred to as “PM sensor”) for detecting an amount of particulate matter included in exhaust gases, a sensor is known that includes an insulating substrate made of ceramics or the like, and a pair of electrodes that are arranged on the insulating substrate so as to face each other with a predetermined space therebetween. The particulate matter is formed of soot or the like that has electrical conductivity. Therefore, when particulate matter accumulates between the pair of electrodes, an electrical current flows between the pair of electrodes via the particulate matter. The PM sensor is configured such that the amount of the electrical current is measured, whereby the amount of the particulate matter included in the exhaust gases is calculated.
However, in the PM sensor described above, in the case where only a small amount of the particulate matter accumulates between the pair of electrodes, because an electrical current path is not formed between the electrodes using the particulate matter, the electrical current does not flow (refer to FIG. 27). In this PM sensor, the electrical current starts to flow after a large amount of the particulate matter accumulates between the pair of electrodes and then the electrical current path is formed between the electrodes using the particulate matter (refer to FIG. 28). Therefore, the PM sensor described above has a problem in that when only a small amount of the particulate matter is accumulated, the electrical current does not flow between the electrodes and it is not possible to detect the particulate matter. That is, there is a problem in that the detection sensitivity for detecting the particulate matter is low.
In order to solve this problem, a PM sensor has been invented in which a pair of electrodes are covered by a conductive part having an electrical resistivity that is higher than the particulate matter (refer to U.S. Pat. No. 7,543,477 as Patent Document 1). In this PM sensor, when the particulate matter accumulates on the surface of the conductive part, the electrical current flows from one electrode to the surface of the conductive surface and passes through the particulate matter having lower resistivity toward the other electrode (refer to FIG. 31). Therefore, even when the amount of accumulated particulate matter is comparatively small, the current flowing between the electrodes changes, so it is considered possible to detect that the particulate matter is accumulated.
However, in Patent Document 1, there is room for improving detection precision of the PM sensor for detecting the particulate matter. In other words, the PM sensor described above is such that electrodes are covered by the conductive part, so when the particulate matter accumulates on the surface of the conductive part (refer to FIG. 31) and then the electrical current flows through the particulate matter, the electrical current passes through a portion from one of the electrodes to the surface of the conductive part. A portion disposed from one of the electrodes to the conductive part is defined as “portion A”. Therefore, when there is variation of the thickness of the portion A during the manufacturing, an electrical resistance varies, and thus a current between the electrodes varies. As a result, it is not possible to measure accurately an amount of the particulate matter. Accordingly, a PM sensor is desired to be able to further improve the detection accuracy for detecting particulate matter.